Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, NY) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single waveguide, each channel being assigned a particular wavelength. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks.
To provide compatibility of WDM systems with existing networks, it is desirable to convert a signal from a received transmission wavelength from a customer to a specific channel wavelength within the WDM system. This is particularly true in WDM systems employing many channels, often referred to as "dense" WDM, where channel spacings are on the order of one nanometer or less. In assignee's prior patent and patent application, U.S. Pat. No. 5,504,609 and Ser. No. 08/624,269, incorporated by reference above, a series of optical remodulators are used to take input signals from various optical transmitters and output the information onto optical channels within the channel plan of a WDM optical system. These optical systems typically employ one-to-one correspondence between an optical channel input to an optical remodulator and an output channel produced by the optical remodulator. However, when the optical transmitters providing the source signals have substantially different data rates from the optical channels used in a WDM system, it may be desirable to have other than one-to-one correspondence between the input and output optical signals. Further, it may be desirable to alter the bit rate of an incident optical signal to a rate best suited for propagation within a transmission line of a WDM optical system.
Thus, there is a need in the art for improved WDM optical communication systems which can receive incoming optical transmission signals of various data rates and place the information from the transmission signals onto one or more optical channels within a WDM system.